The use of code division multiple access (CDMA) is being proposed for the next generation of cellular telecommunication networks.
Additionally, code division multiple access is also being used in the IS-95 Standard in the USA. CDMA is a direct sequence spread spectrum technique. In a wireless cellular network using CDMA, the mobile terminals in one cell associated with a first base station will use the same frequency as mobile stations in an adjacent cell associated with a second base station. The different mobile stations can be distinguished by the respective base stations as each mobile station will be using a different spreading code.
In U.S. Pat. No. 5,101,501 a CDMA system is described which uses “soft” handoff. With soft handoff, a mobile station is capable of communicating with more than one base station at the same time. This will typically occur when the mobile station is close to the boundary defined between two cells. The signals sent by the mobile stations will be received and processed by both of the base stations. Likewise, the mobile station will receive the same signal from the two base stations. The signals from the two base stations may be combined. The combined signal may provide better quality than the individual signals received by the mobile station.
It has also been proposed that two parallel connections be provided between a mobile station and a base station. One of these connections is for speech whilst the other is for data. The soft handoff scenario described hereinbefore has been proposed in the context of speech connections. The use of soft handoff with data is difficult to successfully implement and so it has been proposed that even if the mobile station is in soft handoff with two or more base stations for speech connections, the mobile station should only communicate with a single base station for the data connection.
The inventor has appreciated that this proposal has problems in certain soft handoff situations. These problems will occur when the two base stations with which a mobile station is in communication are under the control of different radio network controllers. The radio network controllers provide control information to the base stations for forwarding to a mobile station. This information indicates if speech and/or data is present and associated rate information. For successful combining in the mobile station when in soft handoff, this information should not contradict. If one base station under the control of one radio network controller indicates that speech and data are being sent to the mobile station and a second base station under the control of a different radio network controller indicates to the same mobile station that only speech is being sent to the mobile station, the mobile station will not be able to successfully combine the received speech. The mobile station may also assume that there is no data information as the mobile station has only been advised that there is data information by one base station. As this information is only received from one base station, the mobile station may assume that the information from the base station is unreliable and that there is in fact no data.
EP-A-0 577 322 describes a method for call handover in a cellular radio system using soft handover, whereby during the transitional period in which the mobile station switches base station, the mobile station is in communication with both base stations on different channels.